Low rank coals (low grade coals), coals having a high moisture content such as lignite and sub-bituminous coals, are abundant. However, the heating value per unit weight is small, and the transportation efficiency is low. Accordingly, such coal is heated and dried to increase the heating value per unit weight, and also compression-molded to improve the handling.
Meanwhile, the low grade coal thus heated is likely to adsorb water. In addition, a carboxyl group and the like are released from the surface, and radicals and the like are generated on the surface. This increases the surface activity, promoting a reaction with oxygen in air. As a result, spontaneous combustion may occur due to a reaction heat generated by the reaction.
For this reason, for example, the following inactivation treatment is carried out. Specifically, low grade coal is dried and pyrolyzed. After tar is separated from the pyrolyzed coal, the pyrolyzed coal is heated (approximately 150 to 170° C.) in a low oxygen atmosphere (oxygen concentration: approximately around 10%) to partially oxidize the surface of the pyrolyzed coal, thereby decreasing the surface activity of the pyrolyzed coal. By such an inactivation treatment, the coal is reformed to prevent spontaneous combustion of the coal (see, for example, the following Patent Literature 1, etc.).